docs: def clear_cache(self, items=None, topic=EVENT_TOPIC):
          """
          expects event object to be in the format of a session-stop or
          session-expire event, whose results attribute is a
          namedtuple(identifiers, session_key)
          """
          try:
              for realm in self.realms:
                  identifier = items.identifiers.from_source(realm.name)
                  if identifier:
                      realm.clear_cached_authc_info(identifier)
          except AttributeError:
              msg = ('Could not clear authc_info from cache after event. '
                     'items: ' + str(items))
              logger.warn(msg)